The “firewall” system of plant defence in response to various abiotic stresses is a complex system, with participation of both symplasmic and apoplasmic components. In many cases, modifications of the cell walls and intercellular spaces (apoplasm), especially those in the root peripheral parts (epidermis, cortex), are the key stage in defence responses. Along the whole plant body there are several checkpoints that regulate water transport and the movement of solutes, including toxic metals, metalloids and (potentially toxic) nutrients. Checkpoints are known at the root surface, starting from the protection of the root apical meristem by the root cap and the interface between the rhizodermis/root hairs and the soil. Radial transport across the root is regulated at the exodermis and endodermis. Suberin, lignin and other cell wall components occurring in peripheral parts of a root form barriers preventing uncontrolled movement of solutes, water and gasses. A further checkpoint is in the transfer from the symplasmic space to the xylem apoplasmic space. Xylem transport along the plant body is regulated by checkpoints between lateral roots and the main root, and in junctions between the root and the stem. Leaf interconnections with the stem in nodes is another checkpoint in transport processes. The sporophyte/gametophyte junction and the connection between maternal tissues and the embryo are the final, and very important, checkpoints in transport processes.

This work was supported by the Scientific Grant Agency of Slovak Republic VEGA 1/0605/17 and by the Slovak Research and Development Agency under the projects APVV-15-0156 and APVV-17-0164.